1. Field of the Invention
The invention relates to a method of producing a polyolefin comprising polymerizing at least one olefin in the presence of a novel catalyst.
2. Discussion of Background
It is already known to use a catalyst system comprising a transition-metal compound and an organometallic compound for low-pressure polymerization of olefins. A catalyst system containing as one component a reaction product of an inorganic or organic magnesium compound with a transition-metal compound is also known as a highly active catalyst.
Japanese Patent Publication Nos. 15110/1977 and 27677/1977 suggest polymerization methods that can produce polyolefins quite high in impact resistance, in which methods use is made of a catalyst system quite high in activity comprising a catalyst component (A) obtained by reacting magnesium metal with a hydroxylated organic compound or an oxygen-containing organic compound of magnesium, an oxygen-containing organic compound of a transition metal and an aluminum halide and a catalyst component (B) of an organometallic compound.
However, polymer particles obtained in the presence of these catalysts are still not satisfactory in the powder properties because the average particle diameter is small, the particle size distribution is wide, and the proportion of fine particles in the polymer particles is high.
That is, if there are such defects, when a polyolefin is produced, various troubles are caused, for example, in the step of separation of particles from the polymer slurry, in the drying step, in the step of transporting the particles, and in the step of pelletizing particles and in some cases long-term continuous production becomes impossible. Further in the cases wherein a polymer is obtained by multi-stage polymerization, if the particle size distribution of the polymer particles is wide, after the drying stage, classification of the powder is liable to occur in the stage where an additive is added or in the stage of transportation, influencing adversely the quality because the physical properties differ from particle to particle, which cannot be ignored sometimes.
We found that the particle diameter of a polymer could be increased by using a silicon compound in addition to a raw material of a catalyst component (A) disclosed, for example, in Japanese Patent Publication No. 15110/1977 and filed a patent application (Japanese Patent Publication No. 58367/1987), but it could not lead to an improvement in the particle size distribution.
Further, we found an improvement in the particle size distribution by partially reducing a raw material of a catalyst component (A) disclosed, for example, in Japanese Patent Publication No. 15110/1977 mentioned above, i.e., a uniform solution containing magnesium and titanium (hereinafter referred to as Mg-Ti solution) with an organoaluminum compound followed by reaction with a silicon compound and then with an aluminum halide compound and filed a patent application (Japanese Unexamined Patent Publication No. 262802/1985}, but it resulted in that the particle diameter was not great enough. Particularly, when the molecular weight distribution was broadened, that tendency became conspicuous and the catalyst particles crumbled readily in the transportation stage and polymerization stage.
Many measures for improving the shape of particles of a polymer such as above means have long been suggested and a process known as preliminary polymerization is one of them. For example, according to Japanese Unexamined Patent Publication No. 172503/1984, a process is disclosed for obtaining a non-crumbling catalyst composition by preliminarily polymerizing propylene with a linear C.sub.8 -C.sub.18 .alpha.-olefin using titanium trichloride compound. However, if this process is applied to polymerization for obtaining a polyethylene by using a catalyst carried on magnesium used in the present invention, the effect of improving a particle diameter and a bulk density is little. Further, Japanese Unexamined Patent Publication No. 215301/1984 discloses a process wherein preliminary polymerization treatment is carried out using butene-1 or 4-methyl-1-pentene in the presence of a catalyst component comprising magnesium, titanium, and halogen as essential components and an organoaluminum compound catalyst component, optionally followed by additional preliminary polymerization treatment with ethylene, propylene or 3-methyl-1-pentene. However, when the present inventors have studied the process, it has been found that since preliminary polymerization is carried out in the presence of an organoaluminum compound catalyst component, the activity of the catalyst is difficult to control and the purpose cannot be attained adequately.